We have recently demonstrated in 184 non endurance trained non obese men and women from the BLSA with normal maximal exercise ECG's that the age-associated decline in VO2 max was markedly attenuated after VO2 max was normalized for total muscle mass instead of body weight. These results suggested an important role of peripheral (vs central circulatory) factors in the age-related reduction of maximal aerobic performance. A closely related issue is whether the well documented marked increase in maximal oxygen-consumption (VO2 max) of older endurance trained men relative to that of age-matched peers is mediated primarily by central versus peripheral adaptations. We performed radionuclide ventriculography and VO2 measurements during maximal upright bicycle ergometry in 8 endurance trained (mean age = 65 plus/minus 5 yr, treadmill VO2 max = 51 plus/minus 4 ml/kg, min) men.